The manufacture of heat-shrinkable films is well known in the art.
As heat-shrinkable film, the expert in the field means a polymeric film which has the ability to shrink or, if restrained from shrinking, to generate shrink tension within the film.
Heat-shrinkable films are well known in the art and their main field of application is package of food and non-food goods.
The term "film" identifies a flexible thermoplastic sheet with a typical thickness of from about 10 microns to about 150 microns and preferably of from about 12 to about 100 microns. When a packaging film to be used as such in a packaging machine is meant, it will typically have a thickness of from about 10 to about 50 microns and preferably of from about 12 to about 35 microns, while when the film has first to be heat-sealed to itself, converted into a flexible thermoplastic container and then used in a packaging machine in the form of a bag or a pouch where the good to be packaged is introduced, it will have typically a thickness of from about 50 to about 150 microns and preferably of from about 50 to about 100 microns.
Heat-shrinkable films typically have a multi-layered structure comprising olefinic polymers and/or co-polymers of various kind, and the terms "polymer" or "polymeric resin", as herein used, generally include homopolymers, copolymers, terpolymers, block polymers, graft polymer, random polymers and alternate polymers.
The manufacture of the above films may generally be accomplished by extrusion (for single layer films) or coextrusion ( for multi-layer films ) of thermoplastic resinous materials which have been heated to their flow or melting point from one extrusion or coextrusion die in, for example, either tubular or planar (sheet) form. After a post-extrusion quenching to cool by well known systems the relatively thick extrudate is then reheated to a temperature within its orientation temperature range, generally below the crystalline melting point but above the second order transition temperature (glass transition point).
The terms "orientation" or "oriented" are used herein to generally describe the process step and resultant product characteristics obtained by stretching and immediately cooling a resinous thermoplastic polymeric material, which has been heated to an orientation temperature range so as to revise the molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material in order to modify certain mechanical properties to the desired properties, for example shrink tension and orientation release stress.
The term "oriented" is also used herein interchangeably with the term "heat-shrinkable". An oriented (i.e. heat-shrinkable) material will tend to return to its unstretched (unextended) dimension when heated to an appropriate elevated temperature.
In the basic process for manufacturing the film as above, the film, once extruded (or coextruded, whenever the case) and initially cooled by, for example, cascade water or chill roll quenching, is then reheated to within its orientation temperature range and oriented by stretching. When the stretching force is applied in one direction, uniaxial orientation results. When the stretching force is applied in two directions, biaxial orientation results. The stretching to orient may be accomplished in many ways such as for example by "blown bubble" techniques or "tenter framing". These processes are well-known to those skilled in the art and refer to orientation procedures whereby the material is stretched in the cross or transverse direction (TD) and/or in the longitudinal or machine direction (MD). After being stretched, the film is rapidly cooled while substantially retaining its stretched dimension and thus set or lock-in the oriented ( aligned) molecular configuration.
After setting the stretch-oriented molecular configuration, the film may then be stored in rolls and utilized to tightly package a wide variety of items.
The above general outline of manufacturing of films is not meant to be all inclusive, since such processes are well-known to the expert in the art. Examples of these processes are disclosed in Italian patent n. 1163118 and U.S. Pat. No. 4,551,380, both in the name of the applicant; said patents also refers to a number of documents relating to the prior art, for example U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443; 4,048,428; 3,821,182; 3,022,543.
Furthermore, when certain characteristics of the film are to be improved, the polymeric structure may be modified in a well-known way. In particular cases, cross-linking of the polymeric structure can be performed, for example by irradiation or chemically. A general disclosure of cross-linking can be found, among others, in U.S. Pat. No. 4,551,380, assigned to the applicant, issued Nov. 5, 1985.
Cross-linked multi-layered heat shrinkable films are there disclosed and claimed.
Generally, a considerable amount of scrap is generated in the course of the manufacture of heat-shrinkable films, such scraps coming from trimming from roll ends, film breakages, filling custom orders requesting special width, or rolls out of specification. In the tenter frame biaxial orientation step, considerable scraps come also from trimming the film edges in the transversal direction.
Such amount of scraps represents an economical burden and a heavy environmental problem due to the waste of plastic material.
A method of recycling coextruded scraps is disclosed in U.S. Pat. No. 4,877,682, assigned to Amoco Corporation, issued Oct. 31, 1989. This patent discloses laminates containing scraps and further discloses articles of manufacture, particularly cookware. The patent relates to thermoplastic materials, which must have particular characteristics as to stiffness and heat-resistance. Among the many exemplary layer materials, polyolefins are cited, particularly crystalline polypropylene, crystalline polyethylene of low, medium, preferably high density. Crystalline polypropylene is said to be particularly preferred because of its high use temperature. The laminates herein disclosed must be capable of resisting deformation or deflection at cooking temperature. Therefore the background of this patent is distant from the one of the present invention which relates to heat-shrinkable films for packaging use.
The International Application WO 91/17886, in the name of E. I. Dupont De Nemours, published 28 Nov. 1991, discloses a multi-layer heat-shrinkable polymeric film containing recycle polymer.
This document claims a process of coextruding a multi-layer heat-shrinkable film having at least a core layer of thermoplastic polymer sandwiched between two outer layers and coextruding recycle of said film into said core along with said thermoplastic polymer of said core. On p. 2, 1.30, of WO 91/17886 it is clearly stated that radiation involved in making particular heat-shrinkable films prevents scrap from the film from being recycled by melt processing, e.g. extrusion. This teaching is repeated on p. 8, 1. 22, as the scrap must be melt processable, hence the original heat shrinkable film from which the scrap was obtained must be free of crosslinking, such as from radiation, which would prevent melt processing.